The present invention relates to an optical amplifier used in a multiplexed optical transmission path, a method of controlling a multiplexed light output delivered out of the optical amplifier, an optical transmission system using the optical amplifier and a method of monitoring and controlling the optical transmission path for transmission of the multiplexed light output.
In recent years, with the demand for reducing the cost of an optical communication system, a so-called wavelength multiplexing optical transmission scheme has been studied in which two or more kinds of signal light beams of different wavelengths are multiplexed and transmitted through a single optical transmission fiber. Since the optical amplifier has a wide amplifying wavelength band and can afford to perform amplification at low noise, it is suitable for use as an amplifier in the wavelength multiplexing optical transmission. A rare earth added optical fiber constituting the optical amplifier and a semiconductor optical amplifier have each the gain which has wavelength dependency and therefore, there occurs a difference in light output or gain between wavelengths after amplification. Specifically, the inter-wavelength difference is accumulated in the course of multi-stage relay based on optical amplifiers and after the relay, the difference in optical power between wavelengths is extended. As a result, the maximum relay transmission distance of the whole system is limited by a degraded S/N ratio of a wavelength of the multiplexed wavelengths which has the lowest power. Accordingly, it is of importance to provide an optical amplifier which does not cause the light output difference between wavelengths.
Thus, for example, a scheme described as "Flattening of Multi-wavelength Amplifying Characteristics in Optical Fiber Amplifier Using Fiber Mu-factor Control" in The Institute of Electronics and Information and Communication Engineers of Japan, Technical Report OCS94-66, OPE94-88 (1994-11) has been known as a conventional scheme.
The conventional scheme uses a fiber gain controller (AFGC) for monitoring the total output of four signal light beams subject to wavelength multiplexing and controlling the fiber gain such that the output level becomes constant. Through this, the inter-input wavelength difference is made to be 0 dB and the fiber gain is controlled to a constant value of 12 dB so as to minimize the difference between wavelengths. Further, by using an auto-power controller (APC) based on an optical attenuator 58, light loss is adjusted while keeping the fiber gain constant at 12 dB to make the fiber gain spectrum unchanged even when the relay mu-factor is changed.
Typically, in a practical system of wavelength multiplexing transmission, transmitting signal information pieces represented by wavelengths are often independent of each other. In this case, only necessary signals are transmitted and there is a possibility that unnecessary signals are stopped, that is, placed in standby condition.
However, in the conventional scheme in which the total output of multiplexed four signal light beams is monitored and controlled, when the number of multiplexed signal wavelengths is changed, the total output remains unchanged but outputs of signal light beams of individual wavelengths change greatly. With the outputs of signal light beams greatly changed in this manner, there arise problems that signal transmission is adversely affected and that when the outputs exceed a signal transmission distance limit, the signal transmission becomes impossible.
Further, in the conventional scheme in which the total light output is controlled, when the output of one signal light beam decreases, this decrease adversely affects the signal transmission.